1. Field of the Invention
This invention relates generally to the electrical connection of integrated circuit packages to circuit boards. More particularly, the invention relates to the packaging of integrated circuits for the removable mounting of the integrated circuits to host boards. The invention has application to all types of integrated circuits.
2. Prior Art
Integrated circuit packages (ICPs) are substantially divided into two main categories. The cateqories are through-hole technology (THT), and surface mount technology (SMT). Examples of three common generic IC packages using the through hole technology are seen in prior art FIGS. 1a-1c.
In FIG. 1a, a cross-section through a plastic (N-type) dual-in-line package (DIP) is seen. The N type DIP consists of a circuit mounted on a lead frame having a desired number of leads, where the circuit is encapsulated within an electrically nonconductive plastic compound. The leads of the package of FIG. 1a are formed by stamping a flat sheet of lead material to form a lead frame, and bending the leads of the lead frame appropriately after the integrated circuit has been mounted on the lead frame and encapsulated, so that the leads may be inserted and soldered into mounting-hole rows of a circuit board.
A cross-section through a JD ceramic DIP is shown in FIG. 1b. The JD ceramic DIP has a hermetically sealed ceramic package with a metal cap and with side-brazed tin-plated leads. As with the N dual-in-line package, the leads of the JD ceramic DIP are intended for insertion and soldering into mounting-hole rows of a circuit board.
A partially cutaway top view of a GB pin-grid-array ceramic package is seen in FIG. 1c. Like the JD ceramic dual-in-line package of FIG. 1b, the GB pin-grid-array ceramic package is also a ceramic package which is hermetically sealed with a metal cap. However, the "leads" of the GB pin-grid-array ceramic package are typically gold-plated pins which are arranged substantially in two squares which surround the hermetically sealed ceramic package. The pins may be inserted and soldered into corresponding mounting holes of a circuit board, or as described in more detail hereinafter, and as is more commonly the case with GB pin-grid array ceramic packages, the package may be inserted into IC sockets which are inserted and soldered into the circuit board.
Examples of three common generic IC packages using the surface mount technology are seen in prior art FIGS. 2a-2c. In FIG. 2a, a perspective view of a D plastic small outline package is seen. The D plastic small outline package is manufactured in a similar manner to, and corresponds closely to the N dual-in-line package, as it consists of a circuit mounted on a lead frame and encapsulated within a plastic compound. The only difference is that the free ends of the leads, instead of being straight for insertion into holes, are bent through approximately ninety degrees (i.e. "gull-wing" bent) for surface mounting.
A top view of an HQ quad flat package is seen in FIG. 2b. The HQ quad flat package is similar to the D plastic small outline package of FIG. 2a, except that the circuit is hermetically sealed, and the gull-wing bent leads are located on all four sides of the chip.
Turning to FIG. 2c, a partially sectioned side view of a FN plastic chip carrier package (PCC package) is seen. As with the D plastic small outline package, the PCC package consists of a circuit mounted on a lead frame and encapsulated within an electrically nonconductive compound. However, instead of two sets of parallel gull-wing bent leads, the leads are "J" leads which extend around the package in a square configuration. The J leads of the PCC package are surface mount soldered to solder lands.
Common to both the through hole packages and the surface mount packages is that in their basic forms, some kind of soldering process is used to fasten the ICPs to the host board for establishing electrical connection between the IC and the board. From a practical point of view, the soldering is a permanent process.
Although most, if not all, of the surface mount and through hole packages are intended to be soldered into contact with a circuit board, some subsets of the two main categories have been provided with IC sockets. The IC sockets effectively act as an interface or an interlocutor between the ICP and the circuit board. Typically, the IC sockets have contacts having a first portions for individually contacting the leads of the ICP, and second portions, which are typically pins, for insertion into and contact with the circuit board. The IC sockets serve various functions, including avoiding the SMT process on a THT board by putting the SMT package in a socket which is soldered into through holes, and providing the ability to replace the IC with a different IC, which is particularly important in updating ROMs.
While IC sockets serve important functions, they have several drawbacks. First, because of their function, the IC sockets introduce an additional physical connection into each circuit. Instead of a single connection where the pins or leads of the ICP contact the circuit board directly, two connections are required; a first connection on between the pins or leads and the IC socket, and a second connection between the pins or leads of the socket and the circuit board. This is a drawback, as every additional connection is a potential failure point, and thus system reliability is decreased. Second, the IC sockets are relatively costly. Third, special tools are often required to extract the ICPs from the IC sockets, and the ICPs can be damaged during extraction. A fourth drawback is that the IC sockets cause the ICPs to be lifted relative to the circuit board, thereby limiting the proximity in which two boards or the board and an additional object may be located.